Left channel no audible sound and filter mix channels combined with high pass filter. Unexpected result

I have a video with audio that is only heard in the right channel. The left channel has no audible signal even though the peak meter reads -30 DbFS.
The left channel waveform is completely flat.

The right channel shows a value of -22.5 dBFS at this point.

Then I applied the channel mix filter and the -22.5 dBFS value increased to -19.5 dBFS for both channels.

The waveform appears to be the same in both channels.

If I apply the filter copy channel (instead of mixing channels), the result is different. If I apply the channel copy filter (instead of mixing channels) the result is different, not raising the volume and keeping the volume down.

What makes them different? Maybe the non-audible signal from the left channel?

I would like to understand this to understand why this happens when I combine the mix channels filter followed by the high pass filter.

If I sandwich the normalize filter: one pass, between the mix channels filter and the high pass filter, then the audio appears again.

Placing the high pass filter before the channel mix filter also seems to work.

My simple test (filter balance to the right) at the beginning of the filter chain, makes me suspect that the left channel, although not heard, incorporates information that the filters use. Is this true?
Why is this happening?

I appreciate your comments in advance. :smiley:

That would be my expectation. You show that the waveform line is “flat”, but the level meter shows there is sound. That is probably because the level meter is in dB (logrimithic) and so very low values would make the line still look flat. When you mix the soft channel with the loud channel, you get a slightly louder channel. But when you copy, the level of both would be the same as the loudest channel.

If you expect that the left channel is just soft noise, then I think the first thing you could want to do is apply the copy filter before the other filters so that you have removed that low level noise.

Thanks for the reply @Brian. :slightly_smiling_face:
I checked again and the left channel has soft noise.
This is what I saw on that channel.

I don’t even know how this happened when I digitized these videos years ago and I have most videos with the audio on the right channel only.
With this type of noise and applying combining channels, I had problems with several audio filters, so I will use the channel copy filter (right to left) to avoid problems.

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